This invention is concerned with a novel crankcase lubricating composition having rust inhibiting properties in large slow speed marine diesel engines.
One significant development in the shipping field is the trend away from steam turbine propulsion units in favor of large marine diesel engines which are increasingly subjected to hostile environment due to the use of low quality petroleum fuels.
The largest marine diesel engines used for ship propulsion are classified as slow speed marine diesel engines. These engines are unique both in their size and in their method of operation. The engines themselves are massive, the larger units approaching 2000 tons in weight and upwards of 100 feet long and 45 feet in height. Their output can reach 50,000 brake horsepower with engine revolutions ranging from about 100 to 125 revolutions per minute.
The slow speed marine diesel engines are unique in their design. Most notably, the crankcase of the large slow speed single acting 2-stroke crosshead type of engine is completely separate from the combustion zone of the engine, i.e. there is no direct communication between the combustion zone and the crankcase zone of this engine. This has led to the use of two different lubrication systems to lubricate a slow speed marine diesel engine. The cylinders in the combustion zone are lubricated with a highly basic 50 to 100 TBN lubricant which functions to counteract the high acidity generated by the use of a high sulfur residual fuel.
The problems encountered in the crankcase of a slow speed marine diesel engine relate to engine lubricant rust problems and water separation characteristics which are essential in a shipboard environment. The essential requirements for a crankcase lubricant for this engine include a rust inhibitor, an anti-wear additive, an oxidation-corrosion inhibitor and a demulsifying agent to maintain the operability and effectiveness of the lubricant.
The lubricant used in the crankcase of a slow speed cross-headed marine engine must protect the engine parts from rust. The rust originates from the reaction of water on ferrous metal parts and the water finds its way into this portion of the engine either through the products of fuel combustion or contamination from an outside water source which could be either fresh or sea (salt) water. The slow speed crankcase formulation, commercial/lubricant may contain a rust inhibitor for this reason; however, there have recently been several complaints that the product is hazy, and the cause of the haze has been identified as the rust inhibitor. The haze, as measured by Lumetron Turbidity measurements, can vary in severity depending on how the batch of rust inhibitor was formulated. Two samples of commercial lubricants had Lumetron values of 44 and 55 which are considered unsatisfactory. Thus, it is an object of the present invention to provide a rust inhibiting additive which renders crankcase oils clear as well as rust protective of marine engines.